A technology known as “Long-Term Evolution, LTE” has been developed for radio communication in cellular networks. In LTE, different schemes of communication can be used for radio nodes in a cellular network such as Frequency Division Duplex (FDD), TDD and half duplex. In this description, the term “radio node” represents any of a base station belonging to a cellular network and a user equipment operated by a user.
In TDD, a single physical channel can be utilized for both uplink and downlink transmissions which must be separated in time, in a communication between a base station and a user equipment. Therefore, the participating radio nodes are required to change between transmit mode and receive mode according to a predefined radio frame scheme, thus avoiding that uplink and downlink transmissions occur on that physical channel simultaneously. An example of such a scheme is illustrated in FIG. 1. In this example, a radio frame 100 of 10 ms duration is divided into ten sub-frames 0-9 of 1 ms duration each, which can be used for either uplink or downlink transmissions on the same physical channel in a communication. In the Third Generation Partnership Project (3GPP), a number of different uplink-downlink configurations have been defined for sub-frames 0-9 in a radio frame. In FIG. 1, some exemplary arrows are shown within the sub-frames to indicate whether a sub-frame is scheduled for uplink or downlink.
Some of the sub-frames can typically be scheduled for either uplink or downlink such as sub-frames 3, 4 and 6. In this exemplifying figure, an uplink transmission in sub-frame 4 is followed by a downlink transmission in sub-frame 5, implying that the base station must switch from receive mode in sub-frame 4 to transmit mode in sub-frame 5. The user equipment must correspondingly switch from transmit mode in sub-frame 4 to receive mode in sub-frame 5. A single sub-frame 1 may even be divided into a field 102 for a downlink Pilot Time Slot, DwPTS, and a field 104 for an uplink Pilot Time Slot, UpPTS, the fields 102 and 104 being separated by a field 106 denoted Guard Period, GP allowing for the above switch and transition of communication modes. This example thus illustrates that both nodes must change between transmit mode and receive mode in a very accurate and synchronized manner to avoid collisions and disturbances on the physical channel used, particularly between uplink and downlink transmissions.
Different radio nodes, including both base stations and user equipments, transmitting in a cellular network are typically required to be mutually synchronized by locking to a common precise reference, such as a pulse emitted from a Global Positioning System (GPS), in order to use a TDD radio frame scheme without collisions. It is also common that multiple parallel transmit branches and antennas are employed in a radio node, e.g. to achieve benefits such as diversity, improved data bit rate and/or enhanced signal reception quality, where the same signals are transmitted or received in parallel over two or more branches and antennas. Some well-known examples of technologies employing parallel branches and antennas are transmit (TX) diversity, Multiple-Input Multiple-Output (MIMO), Beam Forming (BF) and spatial multiplexing. In order to achieve improved performance by using such multiple branches, it is required that the signals emitted from the different antennas are aligned in time, typically also in phase and amplitude.
A simplified example of using multiple branches and antennas in a radio node is schematically illustrated in FIG. 2. The shown radio node 200 may be a base station or a user equipment. Any commonly used amplifiers and filters are omitted in this figure for clarity.
The radio node 200 comprises a digital radio part 202 and two branches 204 and 206, denoted A and B, which are used for both transmission and reception of signals through respective antennas 204c and 206c depending on the mode of communication which can be changed as said above. In the digital radio part 202, a signal generator 202a generates signals which are injected to and transmitted over both branches A and B simultaneously. The signal generator 202a conventionally includes a digital-to-analogue converter, a modulator and an amplifier, which are not shown in this figure for simplicity. The generated signals are first fed to transmit delay buffers 202b and 202c in the radio part 202, which can be pre-configured to delay the signal in time individually in order to calibrate the radio node for output on the two branches and simultaneous emission from the respective antennas 204c, 206c. 
The signals issued from digital radio part 202 are injected to respective transmit (TX) chains 204a and 206a in the branches A and B, respectively, and the branches A and B correspondingly comprise receive (RX) chains 204b and 206b for reception of signals through each of the branches. Each chain can be turned on and off, which is used for changing communication mode in each branch. As indicated in the figure, when the TX chains are on the RX chains are off as indicated by full arrows, and correspondingly when the RX chains are on the TX chains are off as indicated by dashed arrows, in accordance with the prevailing radio frame scheme of sub-frames. The functionality for changing between TX and RX mode in the branches is well known and not necessary to describe in more detail here.
When using such multiple transmit branches and antennas, it is important that the signals are emitted at the same time from the antennas 204c and 206c in transmit mode, otherwise reception of signals on one antenna may be disturbed by transmission of signals from the other antenna, which will be explained in more detail below with reference to FIG. 3 and FIG. 4. Simultaneous emission is also needed to achieve the intended benefit of using parallel branches and antennas. Even though only two antennas are shown in FIG. 2, the above-described arrangement is also applicable for any number of transmit branches and antennas which need to be synchronized in time to avoid misalignment errors.
FIG. 3 depicts a curve 300 showing how output power for transmission from an antenna of a radio node, such as antennas 204c and 206c in the above example, changes over time when switching between receive and transmit modes. First, the output power is at an OFF level when in the RX mode. Then at a time t1, transmission is turned on to change into the TX mode and the output power rises up to an ON level which is reached at a time t2. The period from t1 to t2 is thus a transit period from RX mode to TX mode. Correspondingly, at a time t3, the transmission is turned off to change back again into RX mode and the output power decreases down to the OFF level which is reached at a time t4. The period from t3 to t4 is thus a transit period from TX mode to RX mode.
The transit periods t1-t2 and t3-t4 are needed to ramp up and down, respectively, the output power in the radio node according to the shown curve, which can be done during guard periods between uplink and downlink transmissions in the radio frame when no transmission is allowed from either side, such as in the guard period 106 shown in FIG. 1 or between sub-frames 4 and 5. However, if there is a misalignment in the timing of TX and/or RX modes between two or more parallel branches in a radio node, the reception of signals in one branch may be disturbed, or interfered, by a transmission from another branch, thus causing disturbances in the communication. Such a misalignment between transmit branches may also cause severe equipment damages when one branch is still in receive mode and its antenna receives a very strong signal from a closely located antenna of another branch, e.g. of the same radio node or another close radio node, having just changed to transmit mode, or ramping up to transmit mode. The received signal strength in that case may exceed by many times a normal signal strength of signals received from an opposite radio node in normal communication between a base station and a user equipment.
This is schematically illustrated by an example in FIG. 4 where a radio node comprises two radio units, each being similar to the radio node 200 in FIG. 2, having two branches in each radio unit. In this example, transmission from one branch 1B of a first radio unit is delayed in relation to transmission from another branch 1A of the first radio unit, and also in relation to transmission from two branches 2A and 2B of a second radio unit arranged to transmit the same signals, thus causing a misalignment error of Δt between transmission from branch 1B and transmission from the other branches 1A, 2A and 2B. This misalignment error results in interference from branches 1A, 2A and 2B, while ramping up to the transmit mode during period t1-t2, to branch 1B being still in receive mode during period t1-t2, as indicated by a dashed arrow on the left side in FIG. 4. Correspondingly, branch 1B ramps down from the transmit mode after t4 causing interference to branches 1A, 2A and 2B having already entered receive mode at t4, as indicated by another dashed arrow on the right side in FIG. 4.
It is currently a requirement in 3GPP that the misalignment error between two parallel transmit branches should not exceed a preset limit of 65 nanoseconds to avoid communication disturbances or equipment damages. Therefore, radio nodes are carefully calibrated, e.g. by means of transmit delay buffers coupled to the transmit branches, to fulfill the above requirement. It may still happen that a branch can alter its signal propagation time and/or mode switching, e.g. due to damage or ageing of components, or malfunction of software, such that the resulting misalignment error exceeds the preset limit which may typically go unnoticed, still resulting in a degradation of performance in the radio node. This performance degradation may cause decreased accuracy in signal detection, decreased data throughput, increased interference, radio coverage reduction, severe equipment damages, and so forth.